Conventionally, a vehicle such as a dump truck is provided with a retarder to prevent an overrun of an engine in going down a long hill with a heavy load on. The retarder is controlled by a retarder control system. The retarder control system detects revolution speeds of an engine output shaft and a transmission input shaft. When the detected revolution speeds of the shafts reach a predetermined value, the retarder control system activates the retarder to prevent the overrun of the engine (e.g., cf. Patent Document 1).
[Patent Document] JP-A-10-198417 (FIG. 2, p. 2)